1. Field of the Invention
The present invention relates in general to underground liquid storage tanks and to methods for constructing such tanks.
2. Description of the Prior Art
The following patents were found during a preliminary patentability search directed to class 220, subclasses 445, 447, 448 and 3: U.S. Pat. No. 3,412,891; U.S. Pat. No. 3,472,632; U.S. Pat. No. 3,661,294; U.S. Pat. No. 3,667,423; U.S. Pat. No. 3,851,611; and U.S. Pat. No. 4,374,478. None of the above patents disclose or suggest the present invention.
There are a number of underground tank storage systems now in use, some of which are as follows:
a. The oldest, simplist, and most numerous system in current service is the simple steel tank produced by a number of steel tank manufacturers throughout the United States (and throughout the world). These tanks are usually coated with a material which will tend to protect, to some extent, the tank from outside corrosion due to electrolysis, chemical action, ect. A commonly used coating is coal-tar epoxy. It is somewhat effective, but most manufacturers will not guarantee the tank against outside corrosion for more than a year.
b. Another system uses plastic tanks, usually well known "fiberglass" tank, made of some form of epoxy and reinforced chopped glass fibers. These tanks are very effective against outside corrosion as well as internal corrosion which might occur from chemical action. This tank is fairly high in cost, and special installation procedures, because of the lack of strength of the tank as compared with steel, result in additional installation costs. Spillages have resulted because of improper installation causing extreme stresses and cracking of the tank shell. Such failures would not normally occur with the stronger steel tank.
c. A modified system uses steel tanks with a protective plastic coating. The protective coating is usually FRP (fiberglass reinforced plastic), basically the same material used as in the plastic tanks discussed above. This tank is obviously more costly than the simple steel tanks. Possible disadvantages are that, should the fiberglass coating be chipped away from the steel during transport or during the installation, all of the external corrosive factors will be constructed at the one exposed area of steel. This defect would not be detectable in a pressure test of the tank after installation, and would only show up when, and if, the tank should finally become a "leaker". PA0 d. Another system consists of double-wall steel tanks with standard coating on the exterior tank (the standard coating, that is, coal-tar epoxy coating, or equal). The double-wall steel tank is, in effect, a tank within a tank. The inner and outer shells are held together rigidly with welded members, with free passageways for any leakage that might come from the inner tank and leak into the outer shell. This construction normally includes openings in the top ends of the outer shell, usually fitted with 2" pipe, or larger pipe, so that hydrocarbon detector probes may be inserted into the outer shell, at or near the bottom. Leakage from the basic storage tank, that is, the inner tank, is trapped inside the outer shell so that it can be detected. Some of the disadvantages: This is costly construction; the outer shell may corrode, and product may leak out through it, and/or water may come into it from the surrounding excavation. One advantage is that the inner storage tank, and the outer shell may be tested separately to determine their integrity; this does not always hold true, but it does at the time of initial installation. PA0 e. A modified version uses a double-wall steel system, similar to that discussed above but with a corrosion resistant fiberglass-reinforced-plastic or other exterior coating. PA0 f. A similar system consists of a double-wall reinforced fiberglass-plastic (or other plastic materials) tanks. The fiberglass tank manufacturers have come out with their own version of the "tank within a tank". It is a good system, but extremely expensive. The system has far less strength than the double-wall steel system, installation is expensive, and there is always the concern about future cracking due to faulty installation, or due to "flexing" caused by change in elevation of product inside the tank and by change in elevation of exterior water tables. PA0 g. Another system has been referred to as the "Tank in a Bag". This consists of a steel tank encased in an outer cover of plastic material. One manufacturer uses a rather soft plastic which is slipped over the inner steel tank and then sealed, usually by heat-welding. PA0 h. In some areas double-wall containment is achieved by installing the basic storage tank in "vault", usually made of concrete. Not only is this extremely costly, but if the vault should fail, for example should crack, not only would it not serve the purpose of containing leakage, but it would also admit ground water. PA0 i. Another system positions the complete tank, piping and island dispenser units in a large excavation lined with a plastic material. This is obviously a very expensive system. The complete excavation for the tank and piping system is lined with a plastic material. The plastic usually comes in sheets or strips. These sheets or strips must be sealed together in the field, usually by heat-welding, in order to assure that there will be no passage of liquid, either in or out, through the plastic.
This is an excellent system, very strong structurally, corrosion resistant, with good leak-trapping capabilities, but extremely expensive.
Another manufacturer uses a more rigid plastic outer shell, which is usually factory installed, and the plastic outer shell joints are also heat-welded. Field installation usually requires some additional heat-welding. Some disadvantages are that the outer cover is not rigidly separated from, nor is it rigidly attached to, the inner steel tank. Thus, it is not an integral unit, and does not have the overall strength of some of the other systems. The heat-welded joints could be a possible source of future trouble, if there is any sub-standard quality of welding.
The system is very expensive. It is particularly difficult to install in areas which have a high water-table. Sensor "wells", usually 4" to 6" in diameter, are placed at strategic locations inside the excavation liner for the purpose of detecting the presence of hydrocarbons. False indications of leaks can occur when there are product spills, including spillage due to "overfills".
In some "double-wall" tank systems, provisions are made for connecting to "double-wall piping". The inner tank is connected to the basic pump and vent piping. The outer shell is connected to larger piping which surrounds the basic system piping. Thus if there should be a leak in a pump discharge line, or in a vent pipe, liquid would escape, and would drain back through the outer pipe into the outer shell of the double-wall tank system so that they might be detected by the hydrocarbon detector. Some users prefer not to use the double-wall piping, but prefer to use "Line Leak Detectors" for detecting leaks in the pump piping. Pump piping leakage is of particular significance because of potentially large leaks due to the relatively high pressures developed by the pumping equipment. Vent line leaks are not as significant, but must, of course, be considered when designing a system.
None of the above systems disclose or suggest the present invention.